With the rapid advance of information technology and high-tech industries, most of the sophisticated electronic instruments rely on high-quality power supply to maintain normal operations. Uninterruptible power supply serves as a fail-safe power supply that can ensure the reliability of power supply and provide high-quality electricity. Thus far, uninterruptible power supply has become an optimum solution for providing electricity with high-quality and high reliability.
Generally, uninterruptible power supply is mounted between the external power source and the load. When the external power source, such as a commercially available AC power, is able to supply the power required by the load, the uninterruptible power supply can supply power synchronously to the load or convert the commercially available AC power into backup power by an inverter so as to store the backup power in a rechargeable battery. In case that the commercially available AC power is interrupted or abnormal, the uninterruptible power supply can convert the backup power stored in the rechargeable battery into AC power by the inverter and transmit the AC power to the load, thereby ensuring the normal operation of the load.
When the conventional uninterruptible power supply is used to supply power to an inductive load, such as a transformer or an induction motor, the flux of the inductive load will be unbalanced during the positive half-cycles and negative half-cycles as the voltage of the inductive load is agitated. Thus, the phenomenon of flux saturation will occur, which in turn results in inrush current. The inrush current will impair the uninterruptible power supply and trigger the over-current protection mechanism of the uninterruptible power supply. This would cease the operation of the uninterruptible power supply and shut down the load. In this manner, the uninterruptible power supply can not supply power to the inductive load when the commercial available AC power is abnormal or interrupted, and the inductive load can not sustain the operation under such condition.
To date, the solution to address the aforementioned problem is to use an uninterruptible power supply having a rated output capacity being several times of the rated capacity of the inductive load. Or otherwise, a resistance bank can be connected in series with the output end of the uninterruptible power supply. When the inductive load starts operating or when the voltage of the inductive load is agitated, this serially-connected resistance bank can be used to suppress the amplitude of the inrush current.
Nevertheless, these solutions will increase the manufacturing cost of the uninterruptible power supply and aggrandize the volume of the uninterruptible power supply. Moreover, these solutions can only be used to suppress the inrush current for a single load. In case that the uninterruptible power supply is used to supply power to a multiplicity of inductive loads with different capacity and different operating time, these solutions can not effectively address the problems of flux saturation and inrush current.
Hence, the applicants endeavor to develop a voltage and current control methodology for uninterruptible power supply in order to address the aforementioned problems.